One common usage of metering pumps is in the measuring of components of multi-component products, such as paints, resins, and floor coverings, and also in adding small quantities of concentrate to water as in fertilizing and spraying herbicides onto crop fields.
It is readily apparent that with such widely different usages, numerous requirements are encountered as to make very substantial demands upon the capability of the pump.
Although a two-component metering pump has been known previously, inherent construction features clearly limit the usefulness in certain situations. Metering capability has been provided by mechanically limiting the return stroke of the piston. See U.S. Pat. No. 1,612,727. In limiting the return stroke of the piston, the piston encounters a mechanical stop which suddenly absorbs the inertia of the piston, any assemblies connected with it, and the diaphragm-operating hydraulic fluid or oil. The revolving cam will initially lift off the stopped piston and then impact the piston again to suddenly start it into motion. Depending upon the shape of the cam and the time in each cycle at which the cam reengages the piston, the repeated impact of the cam against the piston and the repeated impact of the piston against the stop can create substantial vibration in the pump.
Other single component diaphragm type metering pumps have used other types of mechanisms for varying the pumping rate. For instance, one pump has a port in the piston for suddenly releasing pressure in the cylinder at the end of the pressure stroke, as the piston port passes by the end of a stationary and adjustable rod, see U.S. Pat. No. 3,285,182. Another pump varies the length of stroke of the piston by varying the eccentricity of the driver for the piston, see U.S. Pat. No. 3,374,750. Such varying of eccentricity involves complicated mechanisms; and relying upon relief of developed pressure at the end of the pressure stroke requires a rather complicated mechanism and the developing of vacuum pressures on the return stroke which cause related complicating problems.
Still other diaphragm type pumps show the basic separation between the hydraulic fluid pumped by the reciprocating piston and the other liquid pumped by the diaphragm. Excess pressure under certain circumstances is relieved in various ways in these pumps and of course refilling of the cylinder after such relief is necessary. See U.S. Pat. Nos. 2,578,746; 3,075,468; 3,254,845; and 3,680,981.